In a Long Term Evolution (LTE) system, a User Equipment (UE) in a connected state needs to implement uplink synchronization and downlink synchronization with a base station before sending data to the base station. Here, the uplink synchronization is implemented by executing a random access process (a sending Time Advance (TA) is acquired at the same time), and the downlink synchronization is implemented by measuring a certain cell by the UE, wherein the TA is mainly configured for the UE to determine the time for sending data.
The UE may start an uplink Time Alignment Timer (TAT) after implementing uplink synchronization. If the UE receives the TA sent by the base station before timeout of the TAT, the UE considers that uplink synchronization is kept between the UE and the base station; otherwise the UE considers that it gets out of uplink synchronization with the base station. The UE needs to re-implement uplink synchronization if being required to send data to the base station again after getting out of uplink synchronization. There is only one carrier in a cell in an LTE system, so that there is only one TA.
FIG. 1 is a flowchart showing random access in an LTE system. As shown in FIG. 1, a random access process may be initiated by sending a Physical Downlink Control Channel (PDCCH) order or Medium Access Control (MAC) layer signalling by a base station, as shown in Step 0 in FIG. 1. When a dedicated random access preamble may be allocated to UE by virtue of the PDCCH order or a Radio Resource Control (RRC) signalling, the random access is called non-conflicting random access. In this case, the dedicated random access preamble may be allocated only by the base station, and the dedicated random access preamble may be configured for the UE through the PDCCH order or a handover command. When the UE needs to select a random access preamble, the random access is conflicting random access. In this case, Step 0 is not executed, and instead Step 1 is directly executed.
A random access resource selected by the UE includes the random access preamble, a time and frequency domain resource of a Physical Random Access Channel (PRACH), or the like. A non-conflicting random access process is implemented by allocating the dedicated random access preamble to the UE by a network side without a resolution process of resolving a conflict, so that a processing flow, as shown in FIG. 1, includes two steps, i.e. Step 0˜Step 1. A conflicting random access process, as shown in FIG. 1, includes four steps, i.e. Step 1˜Step 4, wherein Step 3 and Step 4 are executed to resolve a conflict.
After introduction of a Carrier Aggregation (CA) technology, the UE in a connected state may simultaneously communicate with a source base station through multiple Component Carriers (CCs). After introduction of the CA technology, a Primary Cell (Pcell) and a Secondary Cell (Scell) are also introduced, a serving cell identifier may also be configured for each serving cell, the Scell also has an independent Scell identifier, and the Scell identifier of the Scell is the same as a serving cell identifier of the corresponding Pcell. Due to increase of data traffic, the number of Scells may be increased, for example, to 4, and correspondingly, an application condition may be liberalized. For example, Remote Radio Head (RRH) and repeater technologies may be supported in an uplink, and multiple TAs may be introduced to solve a problem if the problem cannot be solved by one TA. In order to facilitate management, serving cells employing the same TA are divided into a TA group, a TA group including a Pcell is called a PTAG, and a TA group without a Pcell is called an STAG. Timing for UE to execute random access on a Pcell in a PTAG is the same as that for executing random access when there is only one TA, and there is only one time for execution of random access on an STAG, i.e. upon receipt of notification from a network side. That is, there is only a conflicting random access process.
At present, due to shortage of spectrum resources and sharp increase of heavy-traffic services of mobile users, a requirement on hotspot coverage with a high-frequency point such as 3.5 GHz becomes increasingly high. In order to improve user throughput and enhance mobility, a scenario where a low-power node is adopted becomes a new application scenario. However, a high-frequency point has great signal attenuation and has a very small coverage. The small cell of the high-frequency point is not co-sited with an existing cell, so that many corporations and operating companies prefer to seek for a novel enhancement solution, one of which is a dual connectivity technology. Under dual connectivity, the UE may keep data connections with more than two base stations at the same time, but a control plane connection only includes a connection with a cell of one base station, such as a macro cell.
At present, there is no technical solution for random access of UE on a small cell under dual connectivity.